The Mystery of High-Energy Cosmic Rays

Hold out your hand for 10 seconds. A dozen electrons and muons just zipped unfelt through your palm. The ghostly particles are what scientists call "secondary cosmic rays" -- subatomic debris from collisions between molecules high in Earth's atmosphere and high-energy cosmic rays from outer space.

This daily shower, which never stops, is a sign of violent events in deep space.

It’s believed that the majority of cosmic rays come from supernova explosions. When massive stars explode they blast most of their material into space. The expanding shock waves can break apart interstellar atoms and accelerate the debris to unimaginably high energies. Other, unknown cataclysmic phenomena may be at work, too, especially for the most energetic cosmic rays.

Eun-Suk Seo, a professor of physics at the University of Maryland says, “Cosmic ray particles with energies as high as 1020 electron volts have been measured on the ground. This is more energy than we have achieved in the most powerful manmade particle accelerators.”

“But how do natural cosmic ray accelerators pump so much energy into these particles? This is one of the biggest mysteries in astrophysics.”

While indications of the energies of cosmic ray particles can be measured from the ground, Seo and colleagues have taken their studies to higher elevations, directly measuring particles from space before they break up in Earth’s atmosphere.

The cosmic ray detector known as CREAM (The Cosmic-Ray Energetics and Mass investigation) has been launched to the stratosphere above Antarctica onboard long-duration helium-filled balloons. By lofting the detector above 99% of Earth’s atmosphere, researchers get a better idea of what cosmic rays are like before they collide with nuclei in the air above the detector. CREAM is able to measure the energy and direction of each incoming cosmic ray particle and identify the particle type by measuring its charge, thereby providing clues to the particles’ origin and acceleration mechanisms.

Since 2004, the team has flown CREAM seven times over Antarctica accumulating more than 191 days of data from altitudes as high as 120,000 feet.

They’re about to go even higher. CREAM is heading for space. A reconfigured CREAM detector is scheduled to travel to the International Space Station in 2017 aboard SpaceX’s Dragon spacecraft on a Falcon 9 rocket. Named “ISS-CREAM,” it will remain installed on the Japanese Experiment Module, also known as Kibo, for at least three years.

Seo says, “The ISS provides an excellent monitoring platform for high-energy cosmic rays. The station allows for longterm monitoring in lieu of multiple limited-duration balloon flights, providing direct, unimpeded access to incoming cosmic rays without atmospheric interference. The longer exposure times on the space station allow for the measurement of higher energies. ISS-CREAM has a goal of measuring the highest energy possible for direct measurement of high-energy cosmic rays.”

She adds, “The mysterious nature of cosmic rays serves as a reminder of just how little we know about our universe.”

For more high-energy science from beyond Earth’s atmosphere, stay tuned to science.nasa.gov.